Aliphatic methyl ketones are naturally occurring compounds that were first discovered in rue (Ruta graveolens) more than a century ago (30) and have since been commonly found in microorganisms, plants, insects, and mammalian cells (10). These compounds have a variety of important natural and commercial roles, including acting as pheromones and natural insecticides in plants (1), or providing scents in essential oils and flavoring in cheese and other diary products (10). Biosynthesis of methyl ketones has been hypothesized to derive from a variety of different biological pathways such as fatty acid β-oxidation or aerobic alkene/alkane degradation (10, 21). However, studies to elucidate the genes and biochemical pathways involved in the synthesis of these compounds have been quite rare until recently. One research group in particular has carried out extensive biochemical and genetic studies in a wild tomato species (Solanum habrochaites) and identified two key genes, methyl ketone synthase I (ShMKS1) and methyl ketone synthase II (ShMKS2), that are essential for methyl ketone synthesis from fatty acid intermediates in this plant (6, 11, 31). ShMKS2, which belongs to the 4-hydroxybenzoyl-CoA thioesterase (4-HBT) family, is hypothesized to hydrolyze a β-ketoacyl-acyl carrier protein (ACP) thioester intermediate to generate a β-keto acid; ShMKS1, an enzyme that belongs to the α/β-hydrolase superfamily, apparently decarboxylates the β-keto acid released by ShMKS2 to yield a methyl ketone (31).
Despite the commercial relevance of methyl ketones and their prevalence in nature, no genes other than ShMKS1, ShMKS2, and At1g68260 (a ShMKS2 homolog from Arabidopsis thaliana), have been recombinantly expressed and shown to be associated with methyl ketone biosynthesis (31). Metabolic engineering of microbes to overproduce methyl ketones merits additional attention, as these compounds could be relevant to the biofuel industry as well as the flavor and fragrance industry by virtue of their highly reduced, aliphatic character. Indeed, a range of other fatty-acid derived compounds have already been successfully synthesized from metabolically engineered microbes for use as biofuels, such as fatty acid ethyl esters (26), alkanes (24), alkenes (5, 18, 22, 28), and n-alcohols (9).